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Let's say we have a conventional two-bladed fixed-pitch propeller. All things being equal, would the same propeller but with half the size -- 50% radius, have 50% of the performance of the first propeller?

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    $\begingroup$ Here the area counts, so half the length means a quarter of the thrust. Less if hub losses are included. $\endgroup$ – Peter Kämpf Jul 20 '18 at 17:13
  • $\begingroup$ @CrossRoads I actually didn't think about the hub at all, but thanks, good point. $\endgroup$ – Mahdi Jul 20 '18 at 17:43
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Would the propeller hub in the middle be the same size? You did say all things being equal, which I read as "unchanged".

Example, 86" total diameter prop, 12" diameter hub, leaving 2 blades that are (86 -12)/2 = 37" from edge of hub to tip of blade, and 74" of working blade length.

Then you reduce the radius to 43" with same 12" hub, so you have (43 - 12)/2 = 15.5" blade from hub to tip, and 31" of working blade length. 31" is less then 1/2 of 74", so you've lost more than 50%.

Assuming the engine can't spin any faster due to the engine governor limiting the RPMs, you're not getting off the ground.

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Short answer. No. In engine test cells, where torque measurement is not a criteria, techs utilize what is called a club propeller. Club props are usually large 4 blade props which would be seen on engines larger than the one being tested. The difference being the club prop has had a significant length of each blade trimmed away. Why? The club prop is very good at absorbing power while generating proportionally much less thrust (Low Efficiency). Hopefully this at least partially addresses your question.

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  • $\begingroup$ Sorry but I don't really understand your point. I guess you would need to increase the number of blades either if a) the length of your props gets too long that the tip of the blades can go supersonic or b) you need better performance and happy to sacrifice some efficiency or c) you would want more clearance around the prop, so by increasing the number of blades you can reduce the length of each blade. So I'm not totally sure how shorter length on a four-bladed prop answers my question. $\endgroup$ – Mahdi Jul 20 '18 at 17:57
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    $\begingroup$ I guess that to better understand the question, we need to know what you mean by 50%, ie same prop with blades cut by 50%? Or a prop of 50% scale? same rpm? different rpm? To what performance metric are you referencing? My answer was floated in order to give the questioner a real world example of a short blade prop for the purpose of intentionally minimizing thrust and absorbing power. $\endgroup$ – Walker Jul 20 '18 at 19:39

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